The present invention relates to a semiconductor integrated circuit device, and particularly to a technique that can be effectively utilized for a dynamic RAM (random access memory) which contains an automatic refresh circuit.
Memory cells in a dynamic RAM are each constituted by a storage capacitor for storing the data in the form of an electric charge, and a MOSFET for selecting the address. In a memory cell formed on a semiconductor substrate, the electric charge stored in the capacitor decreases with the lapse of time due to leakage currents that flow through a pn junction between the capacitor and the semiconductor substrate and a pn junction of the MOSFET. In order to store correct data in a memory cell at all times despite leakage of the electric charge stored in the capacitor, it is necessary to carry out a refresh operation in which the data stored in the memory cell is read out before it is lost, amplified, and written again to the same memory cell.
A known system for automatically refreshing the memory cells in a 64-kilobit dynamic RAM can be represented by an automatic refresh circuit which is disclosed, for example, in the journal "Denshi Gijutsu", Vol. 23, No. 3, pp. 30--33. That is, the dynamic RAM has an automatic refresh function for automatically refreshing a plurality of memory cells in the dynamic RAM when a refresh control signal REF of a predetermined level is applied to an external terminal thereof, and a self-refresh function according to which, when the refresh signal REF is maintained at a predetermined level for a predetermined period of time, a timer circuit contained therein works so that the refresh operation is performed after every predetermined period of time.
In the conventional refresh timer, a predetermined cycling time has been employed such as 4 ms/256 cycles. In a dynamic RAM having good refresh characteristics (i.e. data holding characteristics), however, the data can be kept stored even when a longer cycling time is employed. The inventors therefore have contrived to select the time of the refresh timer depending upon the characteristics of elements.